I purchased the x3 conversion kit from cnc fusion. Generally it is a great kit, but there are a few things you should be informed of before you purchase.
The ballscrew upgrade is a must for this project.
The tolerances on the mill or the bolt on kit (or both) are not right on. You will have to grind, cut etc if you plan on using this kit.
On my mill, things weren't quite right, and I ended having problems with my ballscrews binding. Contact with the company only indicated that indeed I would need to grind and cut metal in order to make it work right. Some of this work was offered for free after shipping costs by the vendor.
I would suggest you not attempt this kit unless you already have access to a metalworking shop - or really know what you are doing.
I ended up selling my mill at significant loss due to the poor matchup of parts. I spent the extra cash for no backlash, and smooth operation. This is not what I ended up with.
I know others have had better experience, but I seem to be the exception to the rule. I admit that my issues could have been due to lack of experience, or knowledge - I will admit this quickly.
Keep in mind the limitations of your skills and abilities before you purchase this kit.